EIGHTY MILLION PEOPLE IN THE UNITED States are breathing unacceptably high levels of air pollution, according to the U.S. Environmental Protection Agency, and automobile emissions are a major culprit. Engineers at Honda took this gloomy statistic as a challenge. "Building on our latest technologies," says Sakuji Arai, leader of Honda's natural-gas group, "I guided my engineering team to design the cleanest internal combustion engine ever made." They were understandably pleased with themselves when they unveiled their prototype, installed in a Civic, last November. There was only one problem: its exhaust was cleaner than the air in some cities. At official inspection stations, tailpipe emissions would simply not register.

Arai's team eventually managed to detect their engine's "nearly unmeasurable" emissions. It suppressed hydrocarbons, nitrous oxides, and carbon monoxide at least 60 times better than the 1997 federal limits for new cars. It even bested California's much stricter targets for ultralow emission vehicles (ULEV), the world's toughest goals, by a factor of ten. In fact, Honda's natural-gas Civic accomplished all this without sacrificing the benefits of good old internal combustion, such as more horsepower and longer driving range, or the lower cost of mass production.

The choice of fuel--natural gas--had much to do with Honda's record-shattering success. Natural gas contains methane and burns more efficiently than gasoline. And since methane contains less carbon than gasoline, it produces less carbon dioxide, a greenhouse gas. But Honda's persistence was also crucial to the project. The company had already managed to meet the strict ULEV standards with an experimental gasoline engine, so it had no practical need to make further improvements. Honda engineers nevertheless wanted to see how close they could get to a true zero-emissions car, and they decided to modify the gasoline engine to accept natural gas. This involved designing new software to manage combustion variables such as fuel-to-air ratio, new fuel injectors and engine parts redesigned for higher compression, two catalytic converters, a completely sealed fuel system throughout the car to eliminate evaporative pollution, and a fuel tank made of fiber composites that is strong enough to hold sufficient compressed gas for 220 miles of "real world" driving.